A typical fin and tube type heat exchanger construction consists of a heat exchanger core having multiple tubes, or multiple rows of tubes, conveying a first heat exchange medium such as a refrigerant, with the tubes normally being perpendicular to the flow of a second heat exchange medium such as air. The rows of tubes pass through multiple substantially parallel fins which are formed of thin plates of heat conducting material such as aluminum. The plates generally lie in planes substantially parallel to the air flow. The fin plates may be flat or of corrugated form so that some convolution portions of the plates are slightly inclined in a first direction to the air flow, and other convolution portions of the plates are slightly inclined in the opposite direction of the air flow.
In the fin and tube type heat exchanger, the first heat exchange fluid flowing inside the tubes is used to heat or cool a second heat exchange fluid passing over fins external of the tubes. In the type of heat exchanger contemplated herein, the second heat exchange fluid is a gaseous medium and is normally air, so the term "air side" is used herein refer to the heat exchange between the fins and the second heat exchange fluid passing thereover. The term "air" is intended to include both atmospheric air and other gaseous fluids acting as the second heat exchange medium. For a fin and tube heat exchanger, the overall heat transfer is largely controlled by the air side heat transfer coefficient and amount of effective air side heat transfer area. The air side heat transfer coefficient is largely controlled by the boundary layer growth along the fin.
When air flows across the fin surface area, the frictional force at the fin-to-air interface causes a thin layer of stagnant air to develop at the leading edge of the fin, and this stagnant air layer grows in thickness in the direction of air flow. This boundary layer has an insulating effect. The thicker the boundary layer, the more it insulates the fin and inhibits heat transfer to or from the fin. The heat transfer coefficient at the leading edge of a flat surface parallel to the air flow is very large but rapidly decreases with distance along the fin in the air flow direction as the boundary layer thickens.
The heat transfer coefficient at the leading edge of a flat surface inclined to the air flow is less than the heat transfer coefficient at the leading edge of a flat surface parallel to the air flow but does not decrease as quickly in the direction of air flow since the inclined flat surface accelerates the air flow overcoming the frictional forces which cause the increasing boundary layer on the surface of the fin. However, an inclined surface, or a combination of inclined surfaces, acts like a blunt object in the path of the air flow and also develops a wake area behind the object. Within the wake area, the heat transfer is significantly reduced due to the lack of fluid motion.
This latter-mentioned characteristic also greatly affects the heat transfer coefficient of fin surface area upstream of a tube in the air flow direction as opposed to an equal fin surface area downstream of the tube in the air flow direction, since the latter is in a stagnant air flow zone. For purposes of the present invention, a distinction is made between a leading fin area upstream of a particular tube and a trailing fin area downstream of the tube in the air flow direction. Of course it is recognized that when there are multiple rows of tubes, the fin material between adjacent tube rows is first a trailing fin area behind the first tube row and a leading fin area in front of the second tube row when considered in the air flow direction, and such terms are used herein for this concept.
When there are combinations of fin surface areas with the intent of having air flow pass therebetween, the near proximity of such fin areas, such as the leading edge of one such area and the trailing edge of an adjacent such area, forms a grid upon which condensate can cling. In other words, the surface tension of a condensate from the air flow, when the heat exchanger is used as an evaporator, can bridge small openings and thus divert air flow away from these openings. For purposes herein, the term "condensate gap" is used to refer to the distance between the trailing edge of one fin surface area and the leading edge of an adjacent fin surface area in close proximity thereto. The bridging of condensate across the condensate gap causes channeling of flow which bypasses certain fin surface area and thus reduces the total heat transfer to or from inclined fin surface areas.
In order to increase the air flow turbulence, and thus reduce the boundary layer effect, it is furthermore known to strike louvers from the fin plates. Such louvers on corrugated fins are taught in U.S. Pat. No. 4,434,844, issued Mar. 6, 1984 to Sakitani et al and U.S. Pat. No. 4,469,167, issued Sep. 4, 1984 to Itoh et al, wherein the louvers are flat, or in U.S. Pat. No. 4,300,629, issued Nov. 17, 1981 to Hatada et al, wherein the louvers are chevronshaped with one leg of the louvers lying in the plane of the fin convolution. It is noted that, in the latter reference, the louver leg lengths are equal, which reduces the maximum permissible condensate gap, which acts as a condensate trap between the leading and trailing edges of adjacent fin louvers. U.S. Pat. No. 3,265,127, issued Aug. 9, 1966 to Nickol et al, teaches a flat fin plate, which is not used in the typical tube and fin type construction referred to above, wherein the louvers of unequal leg length with the short leg lying in the plane of the fin plate but not necessarily in the orientation which provides the most effective use thereof or provides symmetry for reversibility of air flow direction while maintaining high utilization of fin effectiveness.